SANFORD, Fla. (AP) — The mother and brother of Trayvon Martin testified Friday that screams for help captured in the background of audio of a neighbor's 911 call came from the 17-year-old.

Sybrina Fulton and Jahvaris Fulton took the witness stand during the trial of George Zimmerman, the former neighborhood watch volunteer charged with second-degree murder in Martin's shooting death. Zimmerman has pleaded not guilty and says he shot the teen in self-defense during a fight.

After the audio was played, prosecutor Bernie de la Rionda asked Sybrina Fulton, "Who do you recognize that to be?" She replied, "Trayvon Benjamin Martin."

Identifying the screams on the 911 call could be critical to the case because it may help determine who was the aggressor during the scuffle between Zimmerman and Martin.

Under cross-examination, defense attorney Mark O'Mara suggested that Sybrina Fulton may have been influenced by others who listened to the 911 call, including relatives and her former husband.

O'Mara also asked Fulton hypothetically whether she would have to accept it was Zimmerman yelling for help if the screams did not come from her son. He also asked if she hoped Martin didn't do anything that led to his death.

"I heard my son screaming," Fulton said. "I would hope for this to never have happened and he would still be here."

Jahvaris Fulton, Martin's brother, also testified the screaming voice was that of Martin.

But O'Mara asked him why last year he had told a reporter that he wasn't sure if the voice belonged to Martin. Jahvaris Fulton, in explaining his comment to the reporter, told O'Mara he was "shocked" when he heard it.

"I didn't want to believe it was him," Jahvaris Fulton said.

O'Mara asked to play the television interview for jurors, but Judge Debra Nelson denied his request for the time being.

Jahvaris Fulton's testimony was broken up by efforts to unlock the evidence room adjacent to the courtroom. Unable to open it, court officials called a locksmith with a drill to help them out.

After the mother and brother testified, the doctor who performed an autopsy on Martin took the witness stand. Associate Medical Examiner Shiping Bao started describing Martin as being in pain and suffering after he was shot, but defense attorneys objected. The judge sustained the objection, and Bao was directed away from that line of questioning.

He later estimated that Martin lived from one to 10 minutes after he was shot, and the bullet went from the front to the back of the teen's chest, piercing his heart.

"There was no chance he could survive," Bao said.

Under cross examination, defense attorney Don West questioned why Martin's hands weren't covered in order to preserve evidence on his fingers and why it took three hours to remove the body from the scene. West and Bao talked over each other at several points, requiring the judge to tell everyone to speak one at a time.

With jurors out of the courtroom, Bao acknowledge he had changed his opinion in recent weeks on two matters related to the teen's death — how long Martin was alive after being shot and the effect of marijuana detected in Martin's body at the time of his death.

The associate medical examiner said last November that he believed Martin was alive one to three minutes. He also said Friday that marijuana could have affected Martin physically or mentally, even though he said it didn't last year.

Nelson had ruled before the trial that Martin's past marijuana use couldn't be introduced, and she didn't veer from that ruling on Friday, meaning jurors didn't hear Bao's opinion about the effect of Martin's marijuana use.